Integrated optical versions of the classical Mach-Zehnder optical interferometer are known to the prior art and have found application as an A/D converter, a D/A converter, an astable multivibrator, a bistable device, an optical logic device, and an electromagnetic field sensor.
With reference now to FIG. 1, an integrated optical interferometer is depicted as including a substrate 10 of optically transmissive material and an optical waveguide 12 formed therein by metal diffusion into an etched pattern of a mask formed on a substantially planar surface 10A of the substrate. Waveguide 12 is electrooptic, that is, its optical characteristics substantially change in the presence of an electromagnetic field. Waveguide 12 includes an input branch 12A which diverges into two intermediate (and parallel) branches 12B, 12C, which in turn reconverge into an output branch 12D. Light is coupled to and from the exposed ends of input branch 12A and output branch 12D, respectively, by conventional optical couplers (not illustrated). Metallic electrodes 14 and 16 are formed on the top surface 10A of the substrate on either side of waveguide branch 12B. The application of a voltage across electrodes 14 and 16 varies the optical characteristics of waveguide branch 12B, but not those of waveguide branch 12C. As a result, the phase of the light commonly supplied to waveguide branches 12B and 12C from input branch 12A is differentially varied as it passes through branches 12B and 12C, producing interference at their reconvergence into output branch 12D. Accordingly, the light exiting from output branch 12D is modulated in relation to the magnitude of the voltage applied to electrodes 14 and 16.
The applications of integrated optical interferometers have been limited by the necessity of using metallic electrodes for modulation. As a result, an electromagnetic field cannot be sensed directly by the interferometer. The presence of conductive material on the substrate makes the interferometers vulnerable to various undesired phenomena, such as lightning. The interferometer efficiency, being directly proportional to the length of the electrodes and inversely proportional to the separation therebetween, is limited by currently-available fabrication techniques. The shape of the interferometer is also restricted to the linear, substantially planar form illustrated in FIG. 1 by the necessity of producing the waveguide through the use of metal diffusion and avoiding bends in the waveguide.